It is also beyond the scope of this paper to deal at length with the direct effect of health services on natality, but some discussion is essential since natality control is becoming the prime determinant of population growth. Human reproduction, in a simplified view of a very complicated process, depends upon first, a male and a female capable of reproducing ("natural" fecundity); second, their union (coitus) so that sperm and egg unite; and third, a normal pregnancy of about 10 lunar months. Natality will not occur if one of the partners is not fecund (e.g., sterile from congenital defect, pelvic infection, radiation, surgery, etc.); if union of the partners does not occur (e.g., by delay of marriage or by separation or death of partners); if, although union of the partners occurs, union of sperm and egg are prevented (contraception); or, if pregnancy is terminated by "natural" or induced fetal death.
Health services operate directly to increase natality by improving fecundity and reducing maternal and natural fetal mortality and to decrease natality by making contraception and induced abortion available. The extent to which health services are likely to improve fecundity in LDC's is not adequately known, but the overall effect on natality in most countries is presumed small since fecundity is already high. In regions with high maternal death rates, health services are likely to contribute to lower maternal mortality and thereby increase the number and proportion of fecund couples. Where health services lower the frequency of fetal death, the degree to which natality will increase is uncertain. Apparently the differences between fetal death rates of LDC's and MDC's is relatively small, but LDC underreporting may be very great. Health services that make induced abortion readily and safely available could have dramatic effects in lowering natality in countries where induced abortion is illegal and relatively infrequent now. However, health services' greatest present and potential effect is in increasing the availability and use of effective contraception.
HEALTH SERVICES-DEFINITION AND CLASSIFICATION
The World Health Organization defines health as complete physical, mental, and social well-being. The closer one approaches this ideal, the more difficult it becomes to define it and to classify what activities are needed to achieve it; so for this paper, health services are defined as organized activities intended to decrease the level of human disease and disability. Major components of health services include manpower, facilities, equipment and supplies (all somewhat translatable, for some purposes, into money). Levels of good health (such as physical or mental fitness, reserves, or endurance) are not included here because the difficulties of measuring either good health or the services required far outweigh immediate relevance.
Classifying health services by type of activity and by type of control is useful in understanding the services and in differentiating the effects of popu-